I don't see why extending your life would have to qualify as sinful.
This is speculation: I'm not a Christian.
In Christianity, death brings the judgment of God who sends you to heaven or hell (or purgatory).
If you expect heaven, you don't want to put off death. Suicide is a sin but as long as you don't see non-cryonics as willful suicide, you would want to die early to get to heaven early.
If you expect hell, then you think you've sinned mortally. Most brands of Christianity allow for redemption by various means. If you think you're a sinner, trying to put off death means trying to avoid the judgement of God, which is both just and good; so struggling against it would make you evil. If you fear hell, instead of focusing on avoiding death, you would focus on expiating your sins in order to go to heaven.
In addition, some but not all brands of Christianity have the meme that this world is impure, and one should abstain from it, and not be attached to it. Trying to live longer than is natural is attachment to the profane; one should instead spend their lives thinking of God, praying, abjuring the pleasures of the flesh, etc. in order to obtain heaven.
Hmm. Most of these seem to ignore the fact (not saying YOU are ignoring the fact, but that the religion would have to be ignoring the fact) that there are reasons to extend life that have nothing to do with heaven and hell.
It's interesting that you mention "trying to live longer than is natural is attachment to the profane" - this strikes me as more Buddhist, but I could see Christians believing that, too. However, if cryo is attachment to the profane, so is eating healthy and exercising. Heck, so is eating at all. I am so glad I'm not religious. It causes such horrible cognitive dissonance to harmonize these types of beliefs with other information I have about life.
I heard that women are difficult to convince when it comes to signing up for cryo. In mentioning cryonics to a dying person, there seems to be a consensus that it's not going to happen. I encountered a post: Years saved: Cryonics vs VillageReach, which addressed my main objection (that the amount of money spent on cryo may be better spent on saving starving children, especially considering that you could save multiple children for that amount of money with high probability whereas you save only one life with low probability by paying for cryo). Now I'm open to being persuaded.
My first instinct was to go read a lot about cryo, but it dawned on me that there are a lot of people here who will want to convince family members, some of them female, to sign up - and these people may appreciate the opportunity to practice on somebody. It has been argued that "Brilliant and creative minds have explored the argument territory quite thoroughly." but if we already know all of the objections and have working rebuttals for each, why is it still thought of as extra difficult to get through to women? If there were a solution to this, it would not be seen as difficult. There must be something that pro-cryo people need for persuading women that they either haven't figured out or aren't good enough at yet.
So, I decided to offer myself for experiments in attempting to convince a woman to sign up for cryo and took a poll in an open thread to see whether there was interest. I don't claim to be perfectly representative of the female population, but I assume that I will have at least some objections in common with them and that persuading me would still be good practice for anyone planning to convince family members in the future. Having a study on persuading women would be more scientific but how do you come up with hypotheses to test for such a study if you have no actual experience persuading women?
So, here is your opportunity to try whatever methods of persuasion you feel like with no guilt, explore my full list of objections without worrying about it being socially awkward, (I will even share cached religious thoughts, as annoyed as I am that I still have them.), and I will document as many of my impressions and objections as I can before I forget them.
I am putting each objection / impression into a new comment for organization. Also, I have decided to avoid reading anything further on cryo, until/unless it is suggested by one of my persuaders.
Well, have fun getting inside my head.